Siemens CPU 313C Hardware And Installation Manual page 206

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At the transitions points between lightning protection zones, you must take
measures to prevent surges being conducted downstream.
The principle of the lightning protection zone concept also specifies that all cables
which are capable of carrying lightning current (!) and installed at the transition
points of lightning protection zones must be included in the equipotential bonding
system.
Conductors and cables capable of carrying lightning current are:
• Metal pipes (e.g. water, gas and heat)
• Power cables (for example, mains voltage, 24 V supply)
• Data cables (for example, bus cable).
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The following measures are suitable for lightning protection equipotential bonding
at the transition between lightning protection zones 0 <-> 1:
• Use grounded, spiraled, current-conducting metal straps or metal braiding as a
cable shield at both ends, for example, NYCY or A2Y(K)Y.
• Install cables in one of the following media:
in continuous metal pipes that are grounded at both ends, or
–
in continuously armored concrete ducts or
–
on closed metal cable trays grounded at both ends.
–
Use fiber optic cables instead of metal conductors.
–
12-20
S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03